This invention relates to a marine propulsion device and more particularly to an improved propulsion device for a watercraft that will ensure against the generation of side thrust on the propulsion device regardless of the running condition.
It is well known to mount a marine propulsion device so that it can be trimmed up or trimmed down depending upon the running condition. Normally during high speed running the propulsion device is trimmed up so that the propeller is not submerged as deeply as when running at low speeds or when accelerating. As a result, when the outboard drive is operated in its trimmed up condition, the propeller is only partially submerged while when in its trimmed down condition, the propeller is substantially fully submerged. Although this arrangement provides the desired amount of propeller submersion for the various running conditions, it can cause certain other difficulties.
As is well known, the configuration of the blades of a propeller are such that they give a forward thrust to the watercraft. In addition, due to the configuration of the blades there is also generated, on each individual blade, a side thrust in addition to the forward thrust. When a propeller is fully submerged, the side thrust balance each other due to the fact that there are multiple blades and they are positioned normally diametrically opposite to each other or at least on opposite sides of a plane passing through the center line of the propeller. However, when the outboard drive is trimmed up, only one-half of the propeller is submerged and an uneven thrust is generated that tends to cause a steering force on the outboard drive.
Devices have been proposed that will tend to offset this side thrust which occurs when the outboard drive is trimmed up. Such devices generally configure the skeg or some portion of the lower unit so as to generate an opposing side thrust that will balance that of the propeller when the propeller is not fully submerged. However, these devices by their very nature induce a side thrust which is not balanced when the propeller is fully submerged and hence give rise to forces that tend to cause the outboard drive to be steered under this condition.
It is, therefore, a principal object of this invention to provide a marine propulsion device in which the side thrusts are balanced under all trim conditions of the outboard drive.
It is a further object of this invention to provide a simplified yet effective method for balancing the side thrusts of a marine outboard drive regardless of the running or trimmed condition.
It is a further object of this invention to provide a marine propulsion device wherein the side thrusts are balanced under all running conditions and which does not require the operator's control so as to achieve this balancing.